The human body demonstrates definite proportions. The same is true for proportions of the head. The systematic study of brain proportions and morphometry could start only with the development of in vivo techniques of brain visualisation. Attempts to establish the relationship between landmarks on the skull and anatomical structures within the brain were undertaken starting from the beginning of 19th century. Use of this technique in patients was not practical because of variability in the spatial relationship between sub-cortical structures and cranial landmarks until intracerebral reference points could be located and correlated to sub-cortical target structures. Using a system of coordinates defined based on the brain can help in location of sub-cortical structures. The Talairach and Toumoux (TT) system of coordinates is based on 6 external landmarks and 2 internal landmarks (the anterior commissure—AC and posterior commissure—PC).
The human cerebrum comprises of two hemispheres that exchange information with each other through axons, which are arranged in specific bundles called commissures. The anterior commissure (AC) and posterior commissure (PC) are two such structures. Identification of AC and PC is critical not only because they are important brain structures, but also because their location is crucial in stereotactic and functional neurosurgery [1], localization analysis in human brain mapping [2], medical image analysis [3], structure segmentation and labeling in neuroradiology [4] as well as in registration to reduce the number of degrees of freedom. Major stereotactic brain atlases, such as the Talairach and Toumoux (TT) brain atlas [5], the Referentially Oriented Talairach-Toumoux atlas [6] and the Schaltenbrand-Wahren atlas [7] are based on the AC and PC. The Talairach transformation based on AC and PC is also widely used in human brain mapping for brain comparison across subjects [8]. In addition, the number of references to the TT atlas has been growing exponentially [9].
The AC and PC structures are often hard to detect due to their small size, variability in intensity properties, low data resolution in comparison to their size, presence of neighboring structures with similar appearance (e.g., the fornix, blood vessels) and noise. For neuroanatomy experts, an interactive identification of AC and PC is straightforward for high quality data. However, automation is desirable not only in research but also in clinical practice to increase confidence with which these structures can be identified by a non-neuroradiologist or other non-specialist (e.g., in urgent cases), and to save a substantial amount of time when processing numerous datasets. Extraction of the AC and PC landmarks from T1WI neuroimages is described in [10, 11]. Some spatial properties of cerebral structures and the anterior and posterior commissures were presented in [12, 13]. In paper [12] quantitative and statistical analysis of angles and distances related to the inter-commissural line has been done. In particular the mean length of the inter-commissural line and range were found for 50 subjects. However, none of these results were correlated to the shape of the brain.
In the case that only low-resolution images of the brain are available, a large interslice gap and partial effects might cause partial visibility or non-visibility of the AC and PC landmarks. Non-morphological cerebral images, such as perfusion-weighted images (PWI) (e.g. a CBF map which indicates cerebral blood flow) and diffusion-weighted images (DWI), do not reflect anatomy and this makes it impossible to detect anatomical structures directly. FIG. 1(a) is an example of a CBF image and FIG. 1(b) is an example of a DWI image.